The present invention relates generally to optical communications and, more particularly, to an intra-channel equalizing optical filter for dense wavelength-division multiplexing (DWDM) communications.
Global communication network traffic has been growing exponentially in recent years, mainly driven by the expansion of Internet and volume of new data services which include online video sharing services, such as YouTube, high-definition digital TV, and enterprise storage area network (SAN). As the backbone to provide the transportation pipelines for such traffic volumes, the optical network has received demands for larger bandwidth capacity. The 40 Giga-bit-per-second (Gbps) network is being deployed by network companies worldwide, and the 100 Gbps network has been a hot topic in both telecom and datacom industries in the past year.
In the US, two major telecommunication carriers, Verizon and AT&T, have announced plans to deploy 100 Gbps per channel DWDM service in 2009 and 2010 respectively. IEEE has also formed a high-speed study group (HSSG) to develop the standard for 100 Gb Ethernet. Some carriers are considering bypassing a 40 Gbps stage and upgrading their network from existing 10 Gbps to 100 Gbps. Even higher data rates are also being considered. For example, ITU-T's Study Group (SG) 15 is currently developing the specifications for new optical data unit (ODU) and optical transport unit (OUT) standards. The proposed ODU4/OTU4 rate ranges from 112 Gbps to 174 Gbps.
As channel data rate increases and signal spectrum broadens, the optical signal experiences more physical impairments during transmission, such as chromatic dispersion (CD), polarization mode dispersion (PMD) and fiber nonlinear effects. Another physical impairment is the inter-symbol interference (ISI), which is caused by the spectrum narrowing due to filtering effect from passive optical elements in the transmission path (such as multiplexer, demultiplexer and interleaver). The filtering effect is even more significant at high data rate and narrow channel spacing. For 100 Gbps signal, even with advanced modulation formats and technologies that offer high spectrum efficiency (such as DQPSK, 8PSK, and polarization multiplexing), the signal spectral per channel is still wider than existing 10 Gbps signal, so the ISI cannot be ignored.
There are several existing approaches to mitigate the ISI caused by the filtering effect in DWDM optical transmission system. One technique uses modulation schemes with better spectral efficiency. However, as the data rate increases further, the signal spectrum still grows broader. Another technique uses advanced coding, which is still technically challenging or very expensive at high speed). A different technique uses side band pre-filtering to reduce signal spectral width, which increases system complexity and compromises performance. A further technique uses electronic post-detection processing to equalize the signal, but the high speed electronic is either very expensive or not yet achievable. A yet further technique uses intra-channel optical equalization, but it requires an additional optical component, leading to higher cost, larger footprint and higher optical loss.
Accordingly, there is a need for an improved intra-channel optical equalizing interleaver that suppresses inter-symbol interference ISI caused by the filtering effect to improve the transmission performance of a DWDM signal.